This invention relates to an axial fan with a reversible flow direction.
Modern axial fans are controllable, high-performance machines, which transform mechanical energy into airflow energy. Control options for these machines generally include functions for adjusting the speed of the rotor and changing the setting angle (pitch) of the rotor blade with the aim of adapting the steepness of the lifting force to the current speed and airflow rate.
A level of efficiency in a fan of 90 percent ensures that operating costs are kept to a minimum. However, in addition to design-based efficiency, another potentially important factor is the level of efficiency of the fan, when operating under off-design (part-load) conditions. The most efficient way of regulating the fan is by altering the speed of the rotor.
However, rotational speed control only makes sense, when all the operating points lie near the most energy-efficient system characteristic curve. In cases where these operating points deviate from the most energy-efficient system characteristic curve due to specific system factors (e.g. through the pressure admission of the system, parallel operation with other fans or other, similar factors), it is practical to make changes to both the speed and the pitch of the rotor blades in order to obtain high levels of efficiency for off-design (partial load) performance situations. For this purpose, the rotor blades of the rotor are designed as adjustable elements positioned around a rotational axis.
The rotor can also be combined with an additional subsequent rotor, which converts the kinetic energy of the existing rotating components into static pressure. Aerodynamic efficiency can be significantly improved through the use of a suitable downstream stator corresponding to the rotor. Inlet stators can also be installed in a fan. An inlet stator effects a change in the usable increase in pressure of the fan. The characteristic curve of the fan is raised or lowered depending on the angular momentum generated in front of the rotor (direction of swirl contrary to or equivalent to the rotational direction of the rotor).
When an axial fan of this type is used for an application such as tunnel ventilation, one of the tasks of the fan may involve effecting a change of direction of the airflow, even if this function is only required on an occasional basis. This would apply to fire situations where conflagration gases are to be conveyed against the standard operating direction of flow in order to reach a closer tunnel exit. For the purpose of achieving this change in the direction of airflow, a method is known, whereby the rotating blades of the rotor are turned to a degree, which enables the desired change of flow direction to be effected. This, however, inevitably means that the effectiveness, which can be achieved using this operating format, is drastically reduced, since any downstream stator, which may be present, will now act as an incorrectly positioned xe2x80x9cinlet statorxe2x80x9d, as soon as the flow direction has been changed, and will thereby significantly interfere with the conditions of the airflow to the rotor. Values for aerodynamic performance as well as for energy consumption in relation to the quantity of air conveyed will then be markedly inferior to those achieved under normal operating conditions. Attempts have previously been made to avoid this loss of quality by mechanically rotating the whole fan by 180xc2x0 around an axis perpendicular to the rotational axis of the rotor, whenever a change in the direction of flow is required. This is, however, only really practical for relatively small axial fans.
A possible solution involving thrust reversal (as used in aircraft engines) is rejected on the basis that energy efficient operation is not possible with axial fans in this context. Additionally, this method only involves reverse-flow operations of short duration, whereas the change of direction required in axial fans must be effected for longer periods under the most energy-efficient conditions.
The purpose of this invention is the design of an axial fan with reversible flow direction, according to the type described above, which enables the same aerodynamic performance (in terms of high pressure figures and a high level of efficiency during operation) to be achieved in both directions, with a predetermined volume flow rate.
This objectivexe2x80x94per the intent of this inventionxe2x80x94is accomplished for an axial fan of the type described above through the characteristic specifications contained in claim 1. Preferable embodiments of the invention form the object of the subclaims.
The use of an inlet stator and a downstream stator, combined with the design and adjustability of the guide vanes of these stators, allows the inlet stator to function as a downstream stator and the downstream stator to function as an inlet stator, when the direction of flow is reversed. These guide vanes may be adjusted in the same way as the rotor blades of the rotor, meaning that they can be moved into an optimal position based on the current requirements. Once the airflow has been reversed by adjusting the turning direction of the rotor and/or the blades, it is possible to operate the axial fan in both directions of flow in such a way that the same amount of energy is required for equivalent air movements in both directions of flow and that this energy requirement approximates the energy consumption of those axial fans designed to operate in only one direction without any reversal functionality.
The particular type of application anticipated for the axial fan according to the invention necessitates the use of an adjustable downstream stator. For this reason, an adjustable inlet stator is added to the rotor of this axial fan, which is capable of taking on the function of a downstream stator in the event of an airflow reversal without requiring that the inlet rotor have the capacity to alter the increase in pressure.